The invention relates to composite ceramic superconducting tapes and structures. Tapes including ceramics such as YBa2Cu3O7xe2x88x92xcex4 (YBCO 123), (Pb,Bi)2Sr2Ca2Cu3O (BSCCO 2223), and (Pb,Bi)2Sr2Ca1Cu2O (BSCCO 2212) can become superconducting at relatively high temperatures, e.g., liquid nitrogen temperatures, and are ideal for carrying electrical current over large distances. The composite superconducting tape usually includes superconducting portions of ceramic material within a conductive metal matrix (e.g., BSCCO filaments within a noble metal matrix) or superconducting portions coated on a conductor (e.g., one or more layers of YBCO or BSCCO supported on a conducting substrate). A support structure such as a metallic tape can be laminated to the composite superconducting tape to provide it with mechanical strength and resilience. During operation the superconducting article (e.g., superconducting tape and support structure) is immersed in fluid cryogen (e.g., liquid nitrogen, liquid helium, or supercritical helium) for an extended period of time. During this time fluid cryogen may infiltrate into the superconducting ceramic material. For example, the infiltration may occur when a portion of the ceramic material, which can be porous, is directly exposed to the cryogen, or when one or more surface defects in the composite material provide a channel between the cryogen and the ceramic material.
Such infiltration can be a serious problem because upon warming the article, the cryogen can quickly vaporize, causing pressure to build up within the article. For example, the density of liquid nitrogen at 77 K is seven hundred times greater than that of nitrogen gas at ambient conditions. The pressure build up within the article can create a large physical defect in the superconducting ceramic and significantly degrade its superconducting properties (e.g., transport properties), thus blocking the desired electrical performance of the article. Because the defect introduces the appearance of a bulge or balloon on the exterior of the superconducting article, this problem is referred to as the xe2x80x9cballoonxe2x80x9d problem.
Applicants have discovered that even where composite ceramic superconducting tapes have a metal coating applied to their surface, cryogen may still infiltrate into the ceramic material through porous or microporous defects in the coating and form balloons. Such defects can be difficult to locate prior to balloon formation because they can be exceedingly small and rare along the length of the tape. Thus, a coated tape vulnerable to balloon formation may, to the eye, look perfect prior to cryogenic thermal cycling. Moreover, the likelihood of cryogen infiltration through such defects increases when the fluid cryogen is under pressurized conditions, e.g., up to about 1 to 33 bars, and when the superconducting article is exposed to the fluid cryogen for long periods of time, e.g., several weeks, several years, or many years. Such conditions are typical for superconductive cabling applications.
Applicants have recognized that a surface defect in the composite ceramic superconducting tape can cause an overlapping defect in an applied metal coating. For example, a surface defect may prevent solder from wetting over the defect, thereby causing a microporous defect to form in an applied solder coating. The overlapping defects can provide a channel through which cryogen can infiltrate into the ceramic material. In tapes formed of BSSCO filaments in a noble metal matrix, for example, such surface defects can result from oxides released from the BSSCO powder during the powder-in-tube fabrication of the composite ceramic tape.
More generally, defects in the composite ceramic tape and metal coating can result during handling and applications manufacturing. Microporous defects in the metal coating can also be caused by shrinkage voids during cooling of the metal coating when the corresponding dimensions of the metal coating are too large (e.g., larger than 0.080xe2x80x3). Statistically, some defects in the composite ceramic tape may overlap with defects in the metal coating to form one or more channels through which cryogen can infiltrate into the ceramic material.
Embodiments of the present invention substantially prevent such cryogen infiltration by completely encapsulating the superconducting tape along its length within a sealing structure. The sealing structure hermetically seals the entire surface along the length of the superconducting tape (e.g., the top, bottom, and sides of the tape) from the cryogen bath to prevent cryogen infiltration. For example, in one embodiment, a first stainless steel tape is laminated to the top of the composite ceramic tape and a second stainless steel tape is laminated to the bottom of the composite ceramic tape to sandwich the composite ceramic tape. The stainless steel tapes are selected to be wider than the composite ceramic tape so that they overhang the sides of the composite ceramic tape. Solder fillets can then seal the sides of the ceramic tape because the solder can wet to the overhanging portions of the metallic tapes and form a continuous surface covering the sides of the composite ceramic tape. The combination of the metallic tapes and the solder fillets thus forms the sealing structure.
The sealing structure can generally provide mechanical reinforcement to the composite ceramic tape, e.g., by including one or more metallic laminates. Alternatively, the sealing structure can be separate from such support structure, e.g., it can encapsulate a ceramic tape already having one or more metallic laminates bonded thereto for providing mechanical reinforcement.
In general, in one aspect, the invention features a superconducting ceramic conductor for use in a preselected fluid cryogen including: a composite ceramic superconducting wire having an outer surface along its length; and a sealing structure hermetically surrounding the outer surface to prevent the cryogen from infiltrating into the wire and degrading its superconducting properties.
The superconductor can include any of the following features. The wire and surrounding sealing structure can be greater than 50 meters long. The wire can include a metallic matrix supporting a plurality of superconducting ceramic filaments. Alternatively, the wire can include at least one superconducting ceramic layer and at least one metallic substrate supporting the at least one superconducting ceramic layer. The sealing structure can be metallic. The sealing structure can prevent the cryogen from infiltrating into the wire through the outer surface under pressurized conditions, for example, the pressurized conditions can exceed about 10 atm and the fluid cryogen can be liquid nitrogen.
Furthermore, the wire can be a composite ceramic superconducting tape having a top face, a bottom face, and side faces, and wherein the outer surface is the top, bottom, and side faces. For example, the sealing structure can include: a first metallic tape laminated to the top face of the composite tape; a second metallic tape laminated to the bottom face of the composite tape, the first and second metallic tapes extending beyond the side faces of the composite tape; and non-porous solder fillets adjacent the side faces of the composite tape filling space between the metallic tapes. The metallic tapes can include stainless steel, Cuxe2x80x94Be alloy, aluminum, copper, nickel, or Cuxe2x80x94Ni alloy. The first and second metallic tapes can be at least 5% wider than the composite tape to extend beyond the side faces of the composite tape. The composite tape and the sealing structure can define an aspect ratio for the conductor that is greater than about five. Alternatively, the sealing structure can include: a first metallic tape laminated to the top face of the composite tape and having portions extending beyond the side faces of the composite tape; and a second metallic tape laminated to the bottom face of the composite tape and having portions extending beyond the side faces of the composite tape, wherein adjacent each side face the extended portion of the first metallic tape is welded to the extended portion of the second metallic tape.
In other embodiments, the sealing structure can include a ductile metallic sheet encircling the outer surface of the wire, wherein regions on opposite sides of the metallic sheet are welded to one another. Alternatively, the sealing structure can be a cured polymer layer encircling the outside surface of the wire. In either case, the conductor can further include a metallic tape laminated to the wire for mechanical reinforcement with the ductile metallic sheet or cured polymer layer encircling the wire and the metallic tape. The cured polymer layer can include conductive media, e.g., metallic elements dispersed within the polymer layer. Where the wire has a substantially rectangular cross section, the conductive media can permit the polymer layer to be conductive at least along a direction parallel to the thickness of the wire.
In another aspect, the inventions features a superconducting cable including the superconducting ceramic conductor described above.
In a further aspect, the invention features a superconducting coil including the superconducting ceramic conductor described above.
In a further aspect, the invention features a cryogenically cooled assembly including: a vessel for containing a fluid cryogen; a fluid cryogen; and a superconducting article at least partially immersed in the fluid cryogen, the article including the superconducting ceramic conductor described above in direct contact with the fluid cryogen. In some embodiments, the assembly can further include a refrigeration unit for cooling the fluid cryogen and a circulating pump for passing cryogen through the refrigeration unit. During operation, the circulating pump can cause the pressure of the cryogen fluid in the vessel to exceed 1 atm or even exceed 10 atm.
In general, in another aspect, the invention features a superconducting conductor for use in a preselected fluid cryogen. The conductor includes: a composite superconducting wire having an outer surface surrounding the wire along its length, and a sealing structure hermetically surrounding the outer surface to permit the superconducting ceramic conductor to withstand thermal cycling in which the fluid cryogen is under pressurized conditions without degrading the current carrying capability of the superconducting ceramic tape by more than 10%. For example, the pressurized conditions can exceed about 2 bar (e.g., in the range of about 10 to 33 bar) and the fluid cryogen can be liquid nitrogen.
In general, in another aspect, the invention features a method of making a superconducting conductor for use in a preselected fluid cryogen. The method includes: providing a composite ceramic superconducting wire having an outer surface along its length; and hermetically surrounding the outer surface with a sealing structure to prevent the cryogen from infiltrating into the wire and degrading its superconducting properties.
Embodiments of the method can include any of the following features. The provided wire can be formed by at least one sequence of a mechanical deformation and a subsequent heat treatment of a container including superconducting ceramic precursor. The hermetically surrounding step can include laminating metallic tapes to the wire, encircling at least one metallic sheet around the outer surface of the wire, welding a plurality of metallic sheets to one another to encircle the outer surface of the wire, or forming a polymer coating completely covering the outer surface of the wire. In the latter embodiment, the method can further include adding conductive media to the polymer coating prior to covering the outer surface of the wire.
As used herein, a composite ceramic superconducting wire includes a metallic matrix supporting superconducting ceramic portions, or one or more metallic substrates supporting superconducting ceramic portions. The composite superconducting wire can have an arbitrary cross sectional profile, e.g., a circular, elliptical, or substantially rectangular profile. For example, the composite ceramic superconducting wire can be a composite ceramic superconducting tape.
For the purpose of the present invention, a superconducting wire or tape is meant to describe an elongate composite element capable of carrying current under superconducting conditions, which, after being in contact with cryogenic fluid at superconducting temperatures for a predetermined period of time and subsequently heated to a higher temperature (e.g., room temperature), can show degradation. Such degradation is typically associated with the presence or formation of one or more balloons and/or includes a reduction of the superconducting properties, such as a reduction of the transport critical current.
By way of example, a tape or wire made through a thermo-mechanical process may include a metallic layer on its outer surface, with superconducting ceramic portions formed on the inside. The thermo-mechanical process is capable of causing or facilitating the formation of defects that result in cryogen infiltration and subsequent degradation of the tape or wire.
In another example, a tape or wire includes a layer of superconducting ceramic material applied over a substrate and a surrounding protection layer, typically applied by a sputtering or vaccuum deposition technique. The protection layer, even if effective to protect the superconducting ceramic material from chemical contact with the external atmosphere, has a thickness and strength not sufficient to prevent the cryogenic fluid penetration and the subsequent degradation it causes, particularly when exposed to the cryogenic fluid for a long time or at high pressure.
As described above, the composite ceramic superconducting wire can include a metallic matrix supporting a plurality of superconducting ceramic filaments extending along the length of the superconducting wire. Such a wire can be made by the well-known powder-in-tube process, which involves subjecting a container (e.g., a tube) filled with superconducting ceramic precursor powder to one or more repetitions of a mechanical deformation and heat treatment. Such processing steps can cause defects in the metallic matrix that give rise to cryogen infiltration. Preferably, the sealing structure is formed around the composite ceramic superconducting wire after the wire is made superconducting by the processing steps to avoid exposing the sealing structure to the harsh processing conditions.
Preferably, the metallic matrix is formed from a noble metal. A noble metal is a metal whose reaction products are thermodynamically unstable under the reaction conditions employed relative to the desired superconducting ceramic, or which does not react with the superconducting ceramic or its precursors under the conditions of manufacture of the composite. The noble metal may be a metal different from the metallic elements of the desired superconducting ceramic, such as silver, oxide dispersion strengthened (ODS) silver, a silver alloy or a silver/gold alloy, but it may also be a stoichiometric excess of one of the metallic elements of the desired superconducting ceramic, such as copper.
In another example, the composite ceramic superconducting wire is a multilayer structure including one or more layers of superconducting ceramic, one or more layers of buffer or protection layers, and one or more metal substrate layers supporting the other layers. The multilayer structure can be formed by well-known epitaxy techniques (e.g., sputtering, vacuum deposition, or molecular beam). Although the purpose of the buffer layers is to prevent chemical reactions between the superconducting ceramic and the external environment, such buffer layers are not generally sufficient to prevent cryogen infiltration, particularly when they are exposed to a fluid cryogen for a long time or at high pressure. The sealing structure is formed around the multilayer structure to prevent the cryogen infiltration.
As used herein, xe2x80x9cthermal cyclingxe2x80x9d involves one or more repetitions of three phases in which the superconducting conductor or article is soaked in a coolant bath and returned to room temperature. The three phases are: i) a cooling phase in which the conductor or article is surrounded with coolant, and, optionally, pressure is increased or decreased; ii) a low temperature phase at constant pressure; and iii) a warming phase in which the coolant is removed and, if necessary, pressure is returned to ambient conditions.
Cryogen infiltration of the ceramic material can be determined by inspecting the superconducting conductor or article for balloons after thermal cycling. As used herein, a balloon is a local increase of the composite ceramic wire or tape volume due to internal structure expansion foLlowing thermal cycling. Typically, the volume increase corresponds to a local increase in thickness, e.g., an increase of a few percent to greater than 100% of the total thickness. For example, a balloon can increase the thickness by about 100%. The length of the balloon is a function of the amount cryogen penetration and longitudinal gas diffusion. Balloons have been observed to be about a few millimeters to a few centimeters long, and even longer.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Embodiments of the invention can include many advantages. The sealing structure can prevent cryogen infiltration through surface defects or exposed ceramic surfaces that could otherwise form xe2x80x9cballoonsxe2x80x9d and degrade the critical current carrying capacity of the superconducting wire during the thermal cycling necessary for its normal operation. Prevention of cryogen infiltration through defects in the superconducting composite ceramic wire is crucial to the longevity of the superconducting conductor or article. Formation of even one balloon can end the usefulness of the superconducting conductor or article, for example, because the balloon creates an even larger defect through which cryogen can infiltrate and produce additional balloons upon further thermal cycling. This in turn further reduces the critical current of the superconducting wire. Because of the sealing structure, the conductor can withstand thermal cycling, even in which the fluid cryogen is under pressurized conditions, without degrading the current carrying capability of the superconducting ceramic tape by more than 10% or even less. Prevention of such balloons also preserves the dimensional tolerances of the conductor.
The sealing structure can also prevent cryogen infiltration when the superconductive article is immersed in the fluid cryogen in a pressurized environment (e.g., greater than 1 bar to about 33 bar, such as about 10-15 bar) for long periods of time (e.g., several hours, several weeks, several years, or many years). Such conditions are typical for cabling applications. Moreover, the encapsulation of the composite ceramic superconducting tape provided by the sealing structure can be sufficiently rugged to allow the conductor to be bent or wound into coils or a helix. Furthermore, many embodiments of the superconducting conductor are formed by a continuous process, which allows the formation of long conductors (e.g., longer than about 50 m, and often longer than about several hundred meters).
Other features and advantages of the invention will be apparent from the following detailed description and from the claims.